24 research outputs found
Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)
We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83)
confirming that at least six of the nine merging galaxy pairs identified by van
Dokkum et al. (1999) are indeed bound systems: they have projected separations
of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the
remaining three pairs, we were unable to obtain redshifts of both constituent
galaxies. To identify a more objective sample of merging systems, we select
bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121
confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of
the cluster population. The (B-K_s) color-magnitude diagram shows that the pair
galaxies are as red as the E/S0 members and have a homogeneous stellar
population. The red pair galaxies span a large range in luminosity and internal
velocity dispersion to include some of the brightest, most massive members
(L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members
by z~0.7. These results combined with MS1054's high merger fraction and
reservoir of likely future mergers indicates that most, if not all, of its
early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.Comment: accepted by ApJ Letters; high resolution version of Fig. 2 available
at http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms1054mgrs.ps.g
FIREWORKS U38-to-24 micron photometry of the GOODS-CDFS: multi-wavelength catalog and total IR properties of distant Ks-selected galaxies
We present a Ks-selected catalog, dubbed FIREWORKS, for the Chandra Deep
Field South (CDFS) containing photometry in U_38, B_435, B, V, V_606, R, i_775,
I, z_850, J, H, Ks, [3.6 um], [4.5 um], [5.8 um], [8.0 um], and the MIPS [24
um] band. The imaging has a typical Ks limit of 24.3 mag (5 sigma, AB) and
coverage over 113 arcmin^2 in all bands and 138 arcmin^2 in all bands but H. We
cross-correlate our catalog with the 1 Ms X-ray catalog by Giacconi et al.
(2002) and with all available spectroscopic redshifts to date. We find and
explain systematic differences in a comparison with the 'z_850 + Ks'-selected
GOODS-MUSIC catalog that covers ~90% of the field. We exploit the U38-to-24
micron photometry to determine which Ks-selected galaxies at 1.5<z<2.5 have the
brightest total IR luminosities and which galaxies contribute most to the
integrated total IR emission. The answer to both questions is that red galaxies
are dominating in the IR. This is true no matter whether color is defined in
the rest-frame UV, optical, or optical-to-NIR. We do find however that among
the reddest galaxies in the rest-frame optical, there is a population of
sources with only little mid-IR emission, suggesting a quiescent nature.Comment: Accepted for publication in the Astrophysical Journal, 20 pages, 10
figures, reference to website correcte
Optical Spectroscopy of Distant Red Galaxies
We present optical spectroscopic follow-up of a sample of Distant Red
Galaxies (DRGs) with K 2.3, in the Hubble Deep
Field South, the MS 1054-03 field, and the Chandra Deep Field South.
Spectroscopic redshifts were obtained for 15 DRGs. Only 2 out of 15 DRGs are
located at z < 2, suggesting a high efficiency to select high-redshift sources.
From other spectroscopic surveys in the CDFS targeting intermediate to high
redshift populations selected with different criteria, we find spectroscopic
redshifts for a further 30 DRGs. We use the sample of spectroscopically
confirmed DRGs to establish the high quality (scatter in \Delta z/(1+z) of ~
0.05) of their photometric redshifts in the considered deep fields, as derived
with EAZY (Brammer et al. 2008). Combining the spectroscopic and photometric
redshifts, we find that 74% of DRGs with K 2. The combined
spectroscopic and photometric sample is used to analyze the distinct intrinsic
and observed properties of DRGs at z 2. In our photometric sample
to K < 22.5, low-redshift DRGs are brighter in K than high-redshift DRGs by 0.7
mag, and more extincted by 1.2 mag in Av. Our analysis shows that the DRG
criterion selects galaxies with different properties at different redshifts.
Such biases can be largely avoided by selecting galaxies based on their
rest-frame properties, which requires very good multi-band photometry and high
quality photometric redshifts.Comment: Accepted for publication in the Astrophysical Journal, 13 pages, 8
figures, 5 table
Predicting Quiescence: The Dependence of Specific Star Formation Rate on Galaxy Size and Central Density at 0.5<z<2.5
In this paper, we investigate the relationship between star formation and
structure, using a mass-complete sample of 27,893 galaxies at
selected from 3D-HST. We confirm that star-forming galaxies are larger than
quiescent galaxies at fixed stellar mass (M). However, in contrast
with some simulations, there is only a weak relation between star formation
rate (SFR) and size within the star-forming population: when dividing into
quartiles based on residual offsets in SFR, we find that the sizes of
star-forming galaxies in the lowest quartile are 0.270.06 dex smaller than
the highest quartile. We show that 50% of star formation in galaxies at fixed
M takes place within a narrow range of sizes (0.26 dex). Taken
together, these results suggest that there is an abrupt cessation of star
formation after galaxies attain particular structural properties. Confirming
earlier results, we find that central stellar density within a 1 kpc fixed
physical radius is the key parameter connecting galaxy morphology and star
formation histories: galaxies with high central densities are red and have
increasingly lower SFR/M, whereas galaxies with low central densities
are blue and have a roughly constant (higher) SFR/M at a given
redshift. We find remarkably little scatter in the average trends and a strong
evolution of 0.5 dex in the central density threshold correlated with
quiescence from . Neither a compact size nor high- are
sufficient to assess the likelihood of quiescence for the average galaxy;
rather, the combination of these two parameters together with M
results in a unique quenching threshold in central density/velocity.Comment: 20 pages, 15 figures, and 2 tables; Accepted for publication in the
Astrophysical Journa
Direct measurements of dust attenuation in z~1.5 star-forming galaxies from 3D-HST: Implications for dust geometry and star formation rates
The nature of dust in distant galaxies is not well understood, and until
recently few direct dust measurements have been possible. We investigate dust
in distant star-forming galaxies using near-infrared grism spectra of the
3D-HST survey combined with archival multi-wavelength photometry. These data
allow us to make a direct comparison between dust around star-forming regions
() and the integrated dust content ().
We select a sample of 163 galaxies between with H
signal-to-noise ratio and measure Balmer decrements from stacked spectra
to calculate . First, we stack spectra in bins of
, and find that
, with a significance of
. Our result is consistent with the two-component dust model, in
which galaxies contain both diffuse and stellar birth cloud dust. Next, we
stack spectra in bins of specific star formation rate (),
star formation rate (), and stellar mass (). We
find that on average increases with SFR and mass, but
decreases with increasing SSFR. Interestingly, the data hint that the amount of
extra attenuation decreases with increasing SSFR. This trend is expected from
the two-component model, as the extra attenuation will increase once older
stars outside the star-forming regions become more dominant in the galaxy
spectrum. Finally, using Balmer decrements we derive dust-corrected H
SFRs, and find that stellar population modeling produces incorrect SFRs if
rapidly declining star formation histories are included in the explored
parameter space.Comment: Accepted for publication in the Astrophysical Journal (13 pages, 9
figures
Direct Measurements of the Stellar Continua and Balmer/4000 Angstrom Breaks of Red z>2 Galaxies: Redshifts and Improved Constraints on Stellar Populations
We use near-infrared (NIR) spectroscopy obtained with GNIRS on Gemini,
NIRSPEC on KECK, and ISAAC on the VLT to study the rest-frame optical continua
of three `Distant Red Galaxies' (having Js - Ks > 2.3) at z>2. All three galaxy
spectra show the Balmer/4000 Angstrom break in the rest-frame optical. The
spectra allow us to determine spectroscopic redshifts from the continuum with
an estimated accuracy dz/(1+z) ~ 0.001-0.04. These redshifts agree well with
the emission line redshifts for the 2 galaxies with Halpha emission. This
technique is particularly important for galaxies that are faint in the
rest-frame UV, as they are underrepresented in high redshift samples selected
in optical surveys and are too faint for optical spectroscopy. Furthermore, we
use the break, continuum shape, and equivalent width of Halpha together with
evolutionary synthesis models to constrain the age, star formation timescale,
dust content, stellar mass and star formation rate of the galaxies. Inclusion
of the NIR spectra in the stellar population fits greatly reduces the range of
possible solutions for stellar population properties. We find that the stellar
populations differ greatly among the three galaxies, ranging from a young dusty
starburst with a small break and strong emission lines to an evolved galaxy
with a strong break and no detected line emission. The dusty starburst galaxy
has an age of 0.3 Gyr and a stellar mass of 1*10^11 Msun. The spectra of the
two most evolved galaxies imply ages of 1.3-1.4 Gyr and stellar masses of
4*10^11 Msun. The large range of properties seen in these galaxies strengthens
our previous much more uncertain results from broadband photometry. Larger
samples are required to determine the relative frequency of dusty starbursts
and (nearly) passively evolving galaxies at z~2.5.Comment: Accepted for publication in the Astrophysical Journal. 12 pages, 6
figure
The Color Magnitude Distribution of Field Galaxies to z~3: the evolution and modeling of the blue sequence
Using deep NIR VLT/ISAAC and optical HST/WFPC2 imaging in the fields of the
HDFS and MS1054-03, we study the rest-frame UV-to-optical colors and magnitudes
of galaxies to z~3. While there is no evidence for a red sequence at z~3, there
does appear to be a well-defined color-magnitude relation (CMR) for blue
galaxies at all redshifts, with more luminous galaxies having redder U-V
colors. The slope of the blue CMR is independent of redshift d(U-V)/dMV = -0.09
(0.01) and can be explained by a correlation of dust-reddening with luminosity.
The average color at fixed luminosity reddens strongly \Delta(U-V) = 0.75 from
z~3 to z=0, much of which can be attributed to aging of the stars. The color
scatter of the blue sequence is relatively small sigma(U-V) = 0.25 (0.03) and
constant to z~3, but notably asymmetrical with a sharp blue ridge and a wing
towards redder colors. We explore sets of star formation histories to study the
constraints placed by the shape of the scatter at z=2-3. One particular set of
models, episodic star formation, reproduces the detailed properties very well.
For a two-state model with high and low star formation, the duty cycle is
constrained to be > 40% and the contrast between the states must be a factor >
5 (or a scatter in log(SFR) of > 0.35 dex around the mean). However, episodic
models do not explain the observed tail of very red galaxies, primarily Distant
Red Galaxies (DRGs), which may have ceased star formation altogether or are
more heavily obscured. Finally, the relative number density of red, luminous MV
< -20.5 galaxies increases by a factor of ~ 6 from z = 2.7 to z = 0.5, as does
their contribution to the total rest-frame V-band luminosity density. We are
likely viewing the progressive formation of red, passively evolving galaxies.Comment: 29 pages, 24 figures, in emulateapj style. Abstract is abridged. Some
postscript figures are compressed. accepted for publication in ApJ (scheduled
for August 20, 2007, v665n 2 issue
Measuring the Average Evolution of Luminous Galaxies at z<3: The Rest-frame Optical Luminosity Density, Spectral Energy Distribution, and Stellar Mass Density
(Abridged) We present the evolution of the volume averaged properties of the
rest-frame optically luminous galaxy population to z~3, determined from four
disjoint deep fields with optical to near-infrared wavelength coverage. We
select galaxies above a rest-frame V-band luminosity of 3x10^10 Lsol and
characterize their rest-frame UV through optical properties via the mean
spectral energy distribution (SED). To measure evolution we apply the same
selection criteria to a sample of galaxies from the Sloan Digital Sky Survey
and COMBO-17. The mean rest-frame 2200Ang through V-band SED becomes steadily
bluer with increasing redshift but at z<3 the mean SED falls within the range
defined by ``normal'' galaxies in the nearby Universe. We measure stellar
mass-to-light ratios (Mstar/L) by fitting models to the rest-frame UV-optical
SEDs and derive the stellar mass density. The stellar mass density in luminous
galaxies has increased by a factor of 3.5-7.9 from z=3 to z=0.1, including
field-to-field variance uncertainties. After correcting to total, the measured
mass densities at z<2 lie below the integral of the star formation rate (SFR)
density as a function of redshift as derived from UV selected samples. This may
indicate a systematic error in the mass densities or SFR(z) estimates. We find
large discrepancies between recent model predictions for the evolution of the
mass density and our results, even when our observational selection is applied
to the models. Finally we determine that Distant Red Galaxies (selected to have
J_s - K_s>2.3) in our LV selected samples contribute 30% and 64% of the stellar
mass budget at z~2 and z~ 2.8 respectively. These galaxies are largely absent
from UV surveys and this result highlights the need for mass selection of high
redshift galaxies.Comment: Accepted for publication in the Astrophysical Journal, 24 pages, 16
figure
The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z=0 to z=3
We present the evolution of the rest-frame optical luminosity density, of the
integrated rest-frame optical color, and of the stellar mass density for a
sample of Ks-band selected galaxies in the HDF-S. We derived the luminosity
density in the rest-frame U, B, and V-bands and found that the luminosity
density increases by a factor of 1.9+-0.4, 2.9+-0.6, and 4.9+-1.0 in the V, B,
and U rest-frame bands respectively between a redshift of 0.1 and 3.2. We
derived the luminosity weighted mean cosmic (U-B)_rest and (B-V)_rest colors as
a function of redshift. The colors bluen almost monotonically with increasing
redshift; at z=0.1, the (U-B)_rest and (B-V)_rest colors are 0.16 and 0.75
respectively, while at z=2.8 they are -0.39 and 0.29 respectively. We derived
the luminosity weighted mean M/LV using the correlation between (U-V)_rest and
log_{10} M/LV which exists for a range in smooth SFHs and moderate extinctions.
We have shown that the mean of individual M/LV estimates can overpredict the
true value by ~70% while our method overpredicts the true values by only ~35%.
We find that the universe at z~3 had ~10 times lower stellar mass density than
it does today in galaxies with LV>1.4 \times 10^{10} h_{70}^-2 Lsol. 50% of the
stellar mass of the universe was formed by $z~1-1.5. The rate of increase in
the stellar mass density with decreasing redshift is similar to but above that
for independent estimates from the HDF-N, but is slightly less than that
predicted by the integral of the SFR(z) curve.Comment: 19 pages, 12 figures, Accepted for Publication in the Dec. 20, 2003
edition of the Astrophysical Journal. Minor changes made to match the
accepted version including short discussions on the effects of clustering and
on possible systematic effects resulting from photometric redshift error
What do we learn from IRAC observations of galaxies at 2 < z < 3.5?
We analyze very deep HST, VLT and Spitzer photometry of galaxies at 2<z<3.5
in the Hubble Deep Field South. The sample is selected from the deepest public
K-band imaging currently available. We show that the rest-frame U-V vs V-J
color-color diagram is a powerful diagnostic of the stellar populations of
distant galaxies. Galaxies with red rest-frame U-V colors are generally red in
rest-frame V-J as well. However, at a given U-V color a range in V-J colors
exists, and we show that this allows us to distinguish young, dusty galaxies
from old, passively evolving galaxies. We quantify the effects of IRAC
photometry on estimates of masses, ages, and the dust content of z>2 galaxies.
The estimated distributions of these properties do not change significantly
when adding IRAC data to the UBVIJHK photometry. However, for individual
galaxies the addition of IRAC can improve the constraints on the stellar
populations, especially for red galaxies: uncertainties in stellar mass
decrease by a factor of 2.7 for red (U-V > 1) galaxies, but only by a factor of
1.3 for blue (U-V < 1) galaxies. We find a similar color-dependence of the
improvement for estimates of age and dust extinction. In addition, the
improvement from adding IRAC depends on the availability of full near-infrared
JHK coverage; if only K-band were available, the mass uncertainties of blue
galaxies would decrease by a more substantial factor 1.9. Finally, we find that
a trend of galaxy color with stellar mass is already present at z>2. The most
massive galaxies at high redshift have red rest-frame U-V colors compared to
lower mass galaxies even when allowing for complex star formation histories.Comment: Accepted for publication in the Astrophysical Journal, 16 pages, 16
figure